Solar cells are being utilized to convert light into electricity in which photons going through a semiconductor, release electrons and holes which are then separated by means of an electric field in the semiconductor material. Such solar cells have been known for a long time and are usually made of silicon single crystal wafers. The silicon single crystal wafers are fabricated from silicon single crystal ingots which are pulled by one of the known techniques; subsequently these ingots are sliced into thin slices in the range of several thousandths of an inch up to about .025 inch. These silicon single crystal wafers are then processed which processing is well known from the art. The problem with this approach is that the silicon single crystal material is very expensive. In order to produce inexpensive solar cells one has to use less and less amount of silicon single crystal material. This is, however, limited by the slicing techniques and the general brittleness of the silicon material as it gets thinner. The actual limit today is approximately 0.003 to 0.006 inch thickness, but with the waste caused by the slicing and polishing the minimum amount of silicon utilized to produce one wafer is about 0.006 to 0.020 inch. The major cost in solar cells when the fabrication technology is improved is the price of the silicon single crystal material.
A possibility to reduce the cost of solar cells would be to use inexpensive polycrystalline silicon material instead of single crystal silicon material. Experimental solar cells made of polycrystalline material give very low efficiency. While the terrestrial efficiency of solar cells made of single crystal material is typically on the order of 8 through 18 percent, polycrystalline materials efficiency is on the order of 1%. A relation exists between the crystallinity and the efficiency. It was found that for example, a 5% terrestrial efficiency would require a defect state density of less than 10.sup.18 cm.sup.-3. This means that a silicon material with reasonably good order and with relatively few grain boundaries is needed to obtain useful efficiency. While the utilization of polycrystalline silicon would reduce the cost of the silicon material as it costs a fraction of the single crystal material, the efficiency of these devices because of their polycrystalline nature is low and therefore the practicality of this approach is not economically attractive.
The above means that inexpensive silicon cells will require that single or large crystal silicon must be utilized in the solar cells. A way to reduce the amount of silicon to be utilized is to make solar cells of an extremely thin silicon film in the order of 0.1 - 100 micronmeter. This approach was tried many times in which a thin silicon film was deposited on a substrate, but as of now no successful method has been developed which would produce efficient solar cells this way. While thin films of silicon are usually characterized as polycrystalline, this description covers almost everything between the single crystal and the amorphous state. One of the problems was that deposited silicon material either conforms to the crystal structure of the underlying substrate material or the material deposits in an amorphous state without any crystal structure. The problems originate from the fact that the deposited silicon material grows rather perpendicular to the surface taking up the underlying crystal structure and forms an epitaxial growth. This type of method is utilized many times in semiconductor device manufacturing, but it is detrimental to the solar cell fabrication in which a large area of good crystalline structure is needed. The amorphous silicon is useless for solar cell application. While the thin film silicon deposition is well known it is evident that the order required for thin films to make solar cells was not obtained until now, on a noncrystalline substrate.
From the above it is obvious that a new method is required to produce inexpensive thin film solar cells with high efficiency and good yield.